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Run an aeroacoustic simulation

Bramble predicts aeroacoustic noise with zCFD's Fast Random Particle Mesh (FRPM) method. An acoustic Attitude takes a finished OpenFOAM RANS Attitude in the same Run as its donor, converts its solution, and runs the acoustic solve on it.

Before you start

  • Your Account needs the zCFD methodology and the Donor FRPM workflow. Account admins see a zCFD tag on the Account Settings page when it's enabled. If it isn't, contact support@bramblecfd.com to have it enabled for your Account.
  • You need an OpenFOAM RANS Template Case that works for the geometry, for the donor.

Donor FRPM is a donor workflow, so the rules in use donor workflows apply: the donor is in the same Run, uses the same HPC queue, and the acoustic Attitudes start automatically after it.

Set up the Map

Create a Map with one RANS Attitude, the donor, and one or more acoustic Attitudes. Use only one RANS Attitude per Run: the acoustic pre-check plots use the first RANS Attitude they find.

Create the acoustic Template Case

  1. Create or copy a Template Case with Methodology group Acoustic and Workflows including Donor FRPM.

  2. On the Acoustics tab, fill in the tables:

    Table Fields
    Simulation Settings Case Type (Internal or External), Min Freq and Max Freq (Hz), Output Freq, Order, Sponge Layer Damping Distance (from the far field to where damping starts), Sponge Layer Damping Factor, CFL (sets the local pseudo time step)
    zCFD Domain Base Point (m), Lengths (m)
    zCFD Mesh Refinement STL Refinement (target cell size on the geometry), Global Spacing (largest cell size)
    zCFD Refinement Boxes Name, Level, Min Bounds (m), Max Bounds (m)
    FRPM Domain Box, Translation (m), Rotation (°), Length (m), Lt (m), Constant
    Export Microphone Name, Point (m), Trim (m): the points where acoustic data is recorded
    Export Iso Volumes (Acoustics) ISO Field, Lower Bound, Upper Bound
    Export Movies (Acoustics) Axis (X, Y or Z), Start Position, End Position and Step (m)

    Fill in every Simulation Settings field and both zCFD Domain fields. Bramble only applies the acoustic settings when they're all set.

  3. For an external case, set Case Type to External and use the button beside it to open External Acoustic Case Setup. Under External Microphones, set # Microphones, Radius (m) and Centre (m), then click Save.

  4. Click Save in the header.

To see the FRPM box, microphones, domain and refinement boxes on the model, use the FRPMs, Microphones, zCFD Domain, zMesh Boxes and External Microphones options in the 3D model viewer's Templates tab.

Set up the Run

  1. On the Run's CFD Setup tab, set up the donor: an OpenFOAM Methodology, a standard RANS Workflow and your RANS Template Case.
  2. Set up each acoustic Attitude: Methodology zCFD, Workflow Donor FRPM, and your acoustic Template Case.
  3. Check that they all use the same Queue.

Launch

  1. Launch every Attitude. The acoustic pre-checks need the donor's Pre-Checks to have finished. Until then they fail with "Check failed as donor case Pre-Checks incomplete". If there's no donor, they fail with "Donor recipient case not found".
  2. Review the pre-checks:
    • zCFD Maximum Frequency works out the highest frequency the mesh can resolve: the speed of sound (from the reference temperature) divided by the smallest cell size. It fails with "Template case max frequency > calculated theoretical max frequency: …" if Max Freq is too high. Lower Max Freq or refine the mesh. "Values have not been defined in the template case" means the settings are missing.
    • Acoustic Mesh Plots, FRPM Plots and Microphone Plots show the acoustic mesh, the FRPM box and the microphone positions.
  3. Start Mesh & Solve on the donor. The acoustic Attitudes wait, then start automatically when the donor has finished.

After the solve, Check Max Frequency ZCFD compares the highest frequency the solve's time step can resolve with Max Freq.

View the results

  1. On the Results page, select the acoustic results.
  2. Under Plots, click Acoustics. It's disabled until results are selected.
  3. In Acoustics, use Configure Plot to choose:
    • Attitude
    • Show Microphones from other Attitudes, when more than one Attitude is selected
    • Available Microphones ("Select a Microphone")
    • Available Graphs: PSD (power spectral density, the default) or SPL (sound pressure level)
    • Axis Scale: Log or Linear
    • Plot Type
  4. To download the data, click Download CSV(s). You get one CSV per microphone, named <name>_<data type>_data.csv. "Cannot download CSV due to missing data" means the plotted data isn't available.

Delta post-processing isn't produced for acoustic Attitudes.

Related: Use donor workflows · Template Case editor reference · Checks reference · Export results